1. Field of the Invention
The present invention generally relates to a charging circuit and a charging method used for electronic equipment such as mobile phones, digital cameras, electronic personal organizers, and the like.
2. Description of the Background Art
At present, universal serial bus (USB) host devices are widely used as power sources for loads such as mobile phones. As USB host devices generally have a maximum supply electrical current that the USB host device can supply, it is possible that the USB host device fails to supply electricity if the load connected thereto requires a greater amount of electrical current (hereinafter simply “current”) than the maximum supply electrical current.
To solve this problem, several approaches described below have been proposed.
For example, JP-2006-191796-A proposes a current control circuit like that shown in FIG. 2 for controlling electrical current supplied from a power source having a maximum supply current to the load and a battery.
FIG. 2 is a block diagram illustrating circuitry of a related-art current control circuit 10.
Referring to FIG. 2, the current control circuit 10 includes an input terminal 12 connected to an external power source VBUS, a connection terminal 14 connected to a load 40, and a battery connection terminal 16 connected to a battery 50. Additionally, the input terminal 12 is connected to the connection terminal 14 via a current restriction transistor 24 that is controlled by a current restriction controller 22 as well as to the battery connection terminal 16 via a transistor 35 that is controlled by a charge-current input controller 30. Further, when another AC power source is used, the AC power source is connected to the load 40 via a Zener diode 78, and the battery 50 is charged with charge current IBAT by a charge-current output controller 32 via the transistor 34. The current control circuit 12 further includes a comparator 62 connected to the AC power source via resistors 80 and 82 for detecting the presence of the AC power source. For ease of understanding, operation of the current control circuit 10 when the AC power source is not used is described below.
Referring to FIG. 2, when the external power source VBUS is connected to the input terminal 12, the current restriction controller 22 supplies a load current IOUT to the load 40 via the transistor 24 up to a maximum supply current of the external power source VBUS. Additionally, the charge-current input controller 30 controls the transistor 35, thereby adjusting the amount of the charge current IBAT, to charge the battery 50. To charge the battery 50, the load current IOUT is monitored and the charge current IBAT is reduced so that the sum of the load current IOUT and the charge current IBAT does not exceed the maximum supply current of the external power source VBUS. When the load current IOUT required by the load 40 alone is greater than the maximum supply current of the external power source VBUS, current is also supplied from the battery 50 via an ideal diode 36 to the load 40 because the external power source VBUS alone cannot supply the required amount of current to the load 40.
Additionally, JP2007-60778-A proposes a charging device that uses an USB host device as an external power source and monitors the voltage of the USB host device. This charging device does not charge the battery when the voltage of the USB host device is lower than a predetermined voltage and charges the battery when the voltage of the USB host device is higher than the predetermined voltage.
However, these approaches have several drawbacks.
For example, in the current control circuit 10 according to the first approach, when the amount of current required by the load 40 is greater than the maximum supply current of the external power source BUS and the battery 50 is insufficiently charged, that is, the voltage of the battery 50 is low, the voltage of the battery 50 drops further if current is supplied from the battery 50 to the load 40. Consequently, the load 40 cannot operate because the voltage at the connection terminal 14 to which the load 40 is connected also drops.
By contrast, the charging device according to the second approach does not monitor the current supplied by the external power source. Accordingly, even if the current from the external power source exceeds the maximum supply current, such as USB standards, of the external power source, the amount of current supplied from the external power source cannot be restricted.
In view of the foregoing, the inventors of the present invention recognize that there is a need for a charging circuit and a charging method that restrict the supply current from the external power source to the load within the maximum supply current of the external power source and can prevent the load from becoming inoperative due to an insufficiently charged battery.